Senior Product Engineer, Fullstack in London

About 9fin: 9fin is the AI platform powering global debt markets — the world’s largest asset class at over $145 trillion. Debt markets are vast, global, and mission-critical, yet still run on fragmented data, PDFs, and manual workflows. 9fin replaces this broken infrastructure with a single platform that centralises proprietary credit data, deep analysis, and high-value workflows across global markets. Today, 9fin powers teams at 300+ blue-chip institutions worldwide, including global banks, asset managers, private equity firms, law firms, and advisors. The business is scaling at exceptional speed, with rapid expansion in the US and best-in-class retention driven by deep workflow adoption.

What you'll work on:

• As part of a cross-functional team, lead projects by defining and designing systems, building proof of concepts, and delivering new service functionality - including APIs, data processing and automation - while collaborating with Product and Clients.
• Development and engineering primarily on the backend services and web application hosted tools and systems of the 9fin platform.
• Play an impactful role within project teams as a senior IC, running code reviews, mentoring other developers, leadership & involvement in early research and experimentation with new technologies providing input for new features and products.
• Continually improve the 9fin codebase taking the chance to automate, refactor and document when it’s most impactful.
• Very diverse web platforms, from complex financial data modelling, complex access control business logic, capturing environmental, emission, energy data, incorporation of ML/AI prediction services, data visualisation serving and automation systems.
• Collaboration and teamwork: Cross-functional teams are formed for all projects including the appropriate domain expertise from 9fin financial, legal and editorial colleagues. This allows the best quality information and feedback to go directly to engineering project teams. Your ability to explain and break down complex projects to non-technical audiences will enable productive and efficient team progress.
• Developer Experience: Through managed automated preview environments, feature flags and well-maintained non-production environments. You will be able to move with velocity and get feedback and iterate quickly.

Our Stack:

• Python is the dominant backend language used.
• Flask & FastAPI.
• React & TypeScript.
• Typing is used within our Python stack to power automated documentation and API definitions.
• Relational Databases (PostgreSQL) at the heart of the majority of services.
• We have our own internal developer platform with standardised paved road approaches to infrastructure to bring your code into production as quickly as possible.
• We’re AWS for Cloud hosted services.
• Heavy use of AWS lambda Serverless.
• Docker (AWS ECS) for uniform development/deployment from dev to prod.

Requirements:

• This role will be a great fit if you have experience operating as a Senior IC Engineer, leading systems design and infrastructure discussions, mentoring more junior team members, and scoping work within cross-functional product teams.
• Comfortable moving between frontend and backend work, picking the right tool for the problem rather than specialising in one area.
• Strong knowledge of modern JavaScript/TypeScript and frameworks such as React, alongside experience building scalable backend services and APIs.
• Comfortable leveraging modern AI-assisted development tools (e.g. Cursor, Claude, ChatGPT, GitHub Copilot) to improve productivity while maintaining high engineering quality.
• A strong product mindset with an eye for user experience, working closely with designers and product managers to craft intuitive, polished interfaces.
• Able to independently own features from idea through design, implementation, deployment and iteration.
• Strong system design and problem-solving skills, balancing speed with long-term maintainability.

Benefits:

• We’re a scaling start-up and we enjoy sharing our success; when the company succeeds, we always reinvest that in our people.
• Competitive Salary (our salary bands are benchmarked at the top end of the market).
• Equity options.
• Pension (your minimum contributions are 4% with 9fin matching up to 7%).
• Private Medical Insurance.
• Paid sick leave with Income Protection for long periods of illness.
• Group Life Assurance.
• Season Ticket Loan & Cycle to Work schemes.
• 25 holiday days per year.
• Local public holidays (with the ability to exchange them for alternative days).
• Hybrid working model, to allow you the flexibility to decide how, where and when you do your best work.
• Work abroad for up to 3 months a year.
• 1 month paid sabbatical after 5 years of service.
• Enhanced parental leave & flexible working arrangements available.
• Professional learning and development budget.
• Quarterly team socials.
• Summer and Winter company social events.

9fin is an equal opportunities employer. At 9fin we are dedicated to building and promoting a fair and inclusive workplace where everyone can reach their full potential and truly belong. We recognise that building diverse teams enables a more creative and productive environment. If you’re excited about this role but your experience doesn’t perfectly align with the job description, we encourage you to apply anyway. You might just be who we’re looking for — either for this role, or perhaps another.